the lite version of google voice is used primarily for voicemail and voicemail transcription. if you want sms, you'll have to use the full version of google voice, either requiring you to get a new google number or to port your number to google voice.
if you do port your number out, your line with your phone company will be closed, and you'll have to call them up to reinstate your account but with a different number. the advantage of this is you get the benefits of google voice without having to tell everyone you know that your number changed. however, i think the downsides greatly outweigh the pros. i think google charges $20 for the number porting, then there's the hassle of dealing with the phone company to get your line reinstated, you can't port your number back out (i've read some people trying to limited success), texting from the google voice app is not as nice as texting from the native messaging client, no mms, and more.
my biggest worry is that google will just one day make google voice disappear the same way it's doing to google reader. that uncertainty is reason enough why i'll never port my number out.
